Viral infections may contribute to memory loss

Acute viral infections may cause accumulating damage in the brain and lead to memory problems later in life, a mouse study suggests.

A group of aggressive viruses, including those responsible for common colds, polio and diarrhoea, may harm a crucial memory-processing region in the brain known as the hippocampus, researchers say. The viruses, called picornaviruses, infect more than a billion people worldwide each year – people contract two or three such infections per year on average.

Some people experience a number of acute picornavirus infections in their lives. This fact may explain the severe memory problems seen in some elderly people who do not have neurodegenerative illnesses, such as Alzheimer’s disease, the researchers suggest.

Charles Howe at the Mayo Clinic College of Medicine in Rochester, Minnesota, US, and colleagues injected mice with Theiler’s murine encephalomyelitis virus, which is related to human poliovirus. Because of their genetic makeup, the strain of mice used in the experiment got sick but did not develop paralysis from the infection.

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The team found that even after they had recuperated, the infected mice had problems navigating a maze and remembering a route that they had previously learned, in comparison to uninfected mice.

An analysis of the animals’ brains showed that the mice that had been infected also had severe damage to the memory-processing hippocampus region.

Into the brain

“Our mouse study is the first to show that acute viral infection can cause memory deficit,” says Howe. “We think picornavirus family members cross into the brain and cause a variety of brain injuries. The polio virus can cause paralysis, for example,” he adds.

Howe believes that severe infections known to cause brain inflammation, such as West Nile disease, may cause similar damage to the hippocampus, and subsequent memory problems, in humans.

“If the researchers have found it in mice, there is a chance it would affect humans the same way,” agrees George Kemenes of the University of Sussex in Falmer, Brighton, UK, who was not involved in the study.

Howe’s team has identified a compound that prevents mice from suffering hippocampus damage as a result of viral infection. The compound might help people with viral infections one day, particularly those who suffer from infections that cause brain inflammation.

Because the poliovirus and common cold virus are in the same family of picornaviruses, Howe also speculates that repeated severe colds might possibly cause damage to this brain region that accumulates over a person’s lifetime.

He and his colleagues plan to conduct brain scanning tests to look for signs of such damage in people with a history of acute infections.